1. Field of the Invention
The present invention relates to a display device for displaying an image by modulating light emitted from an optical waveguide plate.
2. Description of the Related Art
Cathode ray tubes (CRTs) are widely used as display devices for personal computers and television receivers. However, liquid crystal displays (LCDs) are rapidly finding widespread use because CRTs are larger in overall size and consume more electric power.
When an LCD displays moving images, it tends to cause an afterimage, also known as ghosting. This phenomenon occurs because the LCD displays images based on a hold-type display principle. The hold-type display principle is a display mode in which an image is displayed substantially continuously during the period of one frame. According to the hold-type display principle, when different images are displayed respectively in two successive frame periods, i.e., when a moving image is displayed, since the images are perceived as being displayed simultaneously in different positions at the instant between the frame periods, an afterimage effect occurs. On the other hand, a CRT displays images based on a non-hold-type (impulse-type) principle. According to the non-hold-type principle, the CRT emits light instantaneously in one frame, and no afterimage occurs since the viewer sees the emitted light instantaneously in each frame.
A process (first process) for improving an afterimage caused by the hold-type display principle will be described below with reference to FIG. 25 of the accompanying drawings. According to this first process, a shutter 1006 is disposed forwardly of a display device (liquid crystal) 1000, at a position closer to an observer 1002, or is disposed rearwardly of the display device 1000 at a position closer to a light source lamp 1004. The display device 1000 displays an image by modulating light from the light source lamp 1004 based on a drive signal from a drive circuit 1008. The shutter 1006 controls transmission of display light from the display device 1000 based on shutter control pulses from a pulse generating circuit 1010 (see, for example, Japanese Laid-Open Patent Publication No. 9-325715).
Specifically, the shutter 1006 turns the light from the light source lamp 1004 on and off, to show an image displayed by the display device 1000 to the observer 1002 only during a certain period of each frame, thus providing a pseudo-impulse display mode.
FIG. 26 of the accompanying drawings shows another process (second process), which does not use a shutter 1006, but wherein control pulses from a pulse generating circuit 1010 are supplied to a power supply 1012 for the light source lamp 1004. According to the second process, electric power supplied from the power supply 1012 to the light source lamp 1004 is turned on and off based on the control pulses to energize and de-energize the light source lamp 1004, thereby showing an image displayed by the display device 1000 to the observer 1002 only during a certain period of each frame. In this case, a pseudo-impulse display mode is also provided (see Japanese Laid-Open Patent Publication No. 9-325715). A lens 1014 serves to focus the image display light from the display device 1000 onto a screen 1016.
As shown in FIG. 27 of the accompanying drawings, there is still another process (third process) for a liquid crystal display device 1024 having an illuminating device 1022 disposed behind a liquid crystal display unit 1020. For preventing displayed moving images from being blurred, the illuminating device 1022 is divided into a plurality of areas (area Za, area Zb, and area Zc), and the illuminating device 1022 is controlled by an illumination driver 1026, to thereby cause the areas of the illuminating device 1022 to separately illuminate the liquid crystal display unit 1020.
The illuminating device 1022 has a light diffusion plate 1028 on an upper portion thereof, which is held in contact with the liquid crystal display unit 1020, and a plurality of lamps 1030 disposed beneath the light diffusion plate 1028. Light reflection plates 1032 are disposed beneath the respective lamps 1030.
The illumination driver 1026 controls energization and de-energization of the lamps 1030 in the three areas, i.e., area Za, area Zb, and area Zc, of the illuminating device 1022 based on a vertical synchronizing signal Sv and a horizontal synchronizing signal Sh from a liquid crystal controller 1034. For example, the illumination driver 1026 controls the lamps 1030 in the three areas of the illuminating device 1022 such that the lamps 1030 are energized after transmittance of the liquid crystal display unit 1020 has become saturated. According to this process, even when the liquid crystal display unit 1020 displays a moving image, by moving a still image at a speed of 10 degrees/second across the visual angle, the observer does not perceive any image blurring whatsoever (see, for example, Japanese Laid-Open Patent Publication No. 2000-275604).
According to yet another process (fourth process: see, Japanese Laid-Open Patent Publication No. 2000-275604), rather than energizing and de-energizing the lamps 1030, a liquid crystal shutter (not shown) is disposed between the lamps 1030 and the liquid crystal display unit 1020, for controlling absorption and transmission of light from the lamps 1030 to achieve an impulse-type display mode in each of plural areas over the liquid crystal display unit 1020, in a similar manner as the third process.
According to the third and fourth processes, since emissions of light from respective areas are successively shifted in synchronism with scanning of the liquid crystal display unit 1020, unlike the impulse-type display modes effected entirely over the display surface (as in the first and second processes), less strict limitations are imposed on the response and emission times of the liquid crystal, allowing the light source to be energized for a longer period of time without the need for increased light emission intensity.
Another process using an illuminating device is disclosed in Japanese Laid-Open Patent Publication No. 2002-49037, for example. As shown in FIG. 28 of the accompanying drawings, the disclosed illuminating device, generally denoted by 1040, has a pair of light guides 1042, 1044 made of acrylic resin, a pair of transparent electrodes 1046, 1048 disposed respectively on lower and upper surfaces of the light guides 1042, 1044, a polymer-dispersed liquid crystal layer 1050 sandwiched between the light guides 1042, 1044, an air layer 1052 disposed on the lower surface of the light guide 1044, a first reflecting plate 1054 disposed below the lower surface of the light guide 1044 with the air layer 1052 interposed therebetween, a light source 1056 disposed on side surfaces of the light guides 1042, 1044, a light source cover 1058 covering the light source 1056, and a second reflecting plate 1060 disposed on opposite side surfaces of the light guides 1042, 1044.
When a voltage of 30 V, for example, is applied to a portion of the polymer-dispersed liquid crystal layer 1050, that portion converts to a transmission phase 1062. When no voltage is applied to another portion of the polymer-dispersed liquid crystal layer 1050, that portion is brought into a diffusion phase 1064. When incident light 1066 from the light source 1056 is applied to the transmission phase 1062, it is not diffused, but propagated through the light guide 1042, and is fully reflected by an interface of the light guide 1042 and travels as reflected light 1068. The reflected light 1068 is fully reflected repeatedly and propagates through the light guides 1042, 1044. When the incident light enters the diffusion phase 1064, it is diffused into diffused light 1070 and 1072. The angle at which the diffused light 1070 (upwardly diffused light) is applied to the interface of the light guide 1042 is smaller than the critical angle, so that the diffused light 1070 is emitted upwardly out of the light guide 1042. The angle at which the diffused light 1072 (downwardly diffused light) is applied to the interface of the light guide 1044 is also smaller than the critical angle, so that the diffused light 1072 is emitted downwardly out of the light guide 1044. Light emitted out of the light guide 1044 is reflected by the first reflecting plate 1054 beneath the illuminating device 1040, enters the illuminating device 1040 again, and is emitted upwardly out of the light guide 1042. Therefore, such light is emitted out of the diffusion phase 1064 of the illuminating device 1040.
By thus controlling transmission and diffusion of light from the light source 1056 using the illuminating device 1040, an impulse display mode is achieved for each of plural areas of the displayed image. Emission of light from each area can thereby be controlled without requiring light absorption.
A display device has also been disclosed having an optical waveguide plate and a plurality of actuators, wherein leakage light from the optical waveguide plate is controlled by the actuators for each of plural areas of the display device (see, for example, Japanese Laid-Open Patent Publication No. 7-287176). As shown in FIG. 29 of the accompanying drawings, the disclosed display device, denoted by 1080, has a light source (not shown) disposed on an end face of an optical waveguide plate 1082, and actuators 1084 are disposed on a surface of the optical waveguide plate 1082 that is opposite to a display surface thereof. When the actuators 1084, or displacement transmitters 1086 connected to the respective actuators 1084, are brought into contact with the optical waveguide plate 1082, light 1088 propagated through the optical waveguide plate 1082 is selectively emitted out of the optical waveguide plate 1082. When the displacement transmitters 1086 connected to the respective actuators 1084 are kept out of contact with the optical waveguide plate 1082, light 1088 is propagated through the optical waveguide plate 1082 by way of total reflection. When one or more of the displacement transmitters 1086 are kept in contact with the optical waveguide plate 1082, light (leakage light) 1090 is emitted from a portion of the display surface of the optical waveguide plate 1082 that corresponds to the displacement transmitter or transmitters 1086 held in contact with the optical waveguide plate 1082. Therefore, light can be emitted from a desired area of the optical waveguide plate 1082 to display an image or the like on the display device 1080.
However, the conventional processes described above suffer the following problems: The display devices disclosed in Japanese Laid-Open Patent Publication No. 9-325715 and Japanese Laid-Open Patent Publication No. 2000-275604 either employ the shutter, or energize and de-energize the light source to achieve an impulse display mode. However, when the shutter turns on and off light, then since the shutter turns off light by blocking it, the efficiency at which the light is utilized tends to be low. Stated otherwise, luminance is lowered, and electric power consumption is increased if luminance is desired to be kept at a constant level. If the light source is energized and de-energized to control the light, then the service life of the light source of the display device, which comprises a cold cathode-ray tube, is shortened. Furthermore, since a light source needs to be provided in association with each area to be turned on and off, the display device is costly to manufacture and large in size. In addition, the light source is utilized with poor efficiency, because it is not used for areas where light is turned off.
The display device disclosed in Japanese Laid-Open Patent Publication No. 2002-49037 does not require a shutter for blocking light and, with its light source turned on at all times, can turn on and off light for each area. The disclosed display device employs a liquid crystal for transmitting and diffusing light. During the transmission phase, in areas where light is guided through the optical waveguide plates by way of total reflection (areas from which no light is emitted), light is transmitted through the liquid crystal. However, since the liquid crystal does not highly transmit light, it tends to cause a loss of light. Although light is transmitted only once or twice through the liquid crystal and loss of light is small, light is fully reflected repeatedly through the optical waveguide plates and thus passes many times through the liquid crystal layer. Since loss of light is commensurate with the number of times that light passes through the liquid crystal layer, light is not used with high efficiency.
Similarly, the display device disclosed in Japanese Laid-Open Patent Publication No. 7-287176 does not require a shutter for blocking light and, with its light source turned on at all times, can turn on and off the light for each area. The disclosed display device is designed to utilize only light that is not fully reflected in the optical waveguide plate, and which is directly applied to the displacement transmitters, and thus, the display device does not effectively utilize light applied to other regions or to the displacement transmitters. In particular, optical components cannot be added to the display surface of the optical waveguide plate, and the disclosed display device does not have the advantages provided by optical components and cannot effectively utilize light.